In optical communication systems, in order to transmit more information on a line of optical fiber, there is used a wavelength division multiplexing. Namely, a frequency domain is divided into a plurality of channels, and a plurality of optical signals respectively corresponding to the plurality of channels are multiplexed to be transmitted on the same optical fiber at the same time.
Here, it is required to monitor that optical powers of optical signals of the respective optical channels are within normal range. An optical channel monitor (hereinafter, referred to as “OCM”) is a device to monitor optical powers of respective channels of an optical circuit. OCMs are largely categorized into two types: a monochromator system and a polychromator system.
An OCM of monochromator system carries out wavelength scan of an optical filter provided internally at first, receives outputs of the optical filter by a photo detector, and monitors optical levels of the received incident light at respective wavelengths. Because of its structure, the OCM of monochromator system requires a light source provided externally in order to secure wavelength accuracy by compensating variation with time of the optical filter. Moreover, since the time to carry out the wavelength scan is required, it takes a comparatively longer time to collect data.
An OCM of polychromator system monitors optical levels of incident light at respective wavelength by arranging a plurality of photo detectors on a demultiplexing side of a wavelength demultiplexer such as a diffraction grating and scanning light reception levels of the respective photo detectors. Since the plurality of photo detectors collect data at the same time, the OCM of polychromator system operates at higher speed than the OCM of monochromator system. However, as for a conventional OCM of polychrometor system, a resolving power is required to be improved in order to distinguish amplified spontaneous emission (hereinafter, referred to as “ASE”) from optical signal components. With reference to the necessity, there is known a problem that a lot of numbers of photo detectors are required to be included in an OCM of polychromator system and then result in increase in its manufacturing cost.
With reference to the above descriptions, Japanese Patent Publication (JP-P2000-292644A) discloses an optical module. The optical module includes a collimator, demultiplexing means and light receiving means. Here, the collimator converts multiple wavelength signal light from an optical transmission line into parallel rays. The demultiplexing means include a plurality of optical filters of different passing wavelength bands and demultiplex the parallel rays into a plurality of signal lights by making the parallel rays to pass the plurality of optical filters. The light receiving means receive the signal lights of respective wavelengths, which are demultiplexed by the demultiplexing means.
Japanese Patent Publication (JP-P2001-168841A) discloses a wavelength multiplex optical amplifier. The wavelength multiplex optical amplifier amplifies at least two or more signal light wavelengths as a bundle. The wavelength multiplex optical amplifier includes an optical monitoring section and an optical variable attenuator section. Here, as for the optical monitoring section, the wavelength multiplex optical amplifier is a fiber type optical amplifier, as for the optical amplifier, impurity material, impurity concentration and fiber length of impurity-doped fiber and excitation intensity are set such that a desired gain and a flat gain profile are obtained when the number of input wavelength is maximum number, and, it is possible to monitor optical levels of wavelength multiplexed light for respective wavelengths and monitor wavelength number of the wavelength multiplexed light. The optical variable attenuator section attenuates optical levels of lights of respective wavelengths without changing gain profile of the wavelength multiplexed light or with planarizing the gain profile. The wavelength multiplex optical amplifier is characterized in that the attenuation of the optical variable attenuator section is determined by detection signals of the optical monitoring section.
Japanese Patent Publication (JP-P2002-319899A) discloses a wavelength monitoring device. The wavelength monitoring device monitors, for respective wavelengths, an optical signal which is wavelength-multiplexed to be transmitted. The wavelength monitoring device includes an interleaver and a plurality of wavelength monitoring circuits. Here, the interleaver divides the wavelength multiplexed optical signal into a plurality of families in accordance with the arrangement of wavelengths. The plurality of wavelength monitoring circuits respectively demultiplex optical signals of the respective families divided by the interleaver and detect optical signals of respective wavelengths.